shiny/driver/gldriver: (X11) calculate PixelsPerPt.

This matches the x11driver.

Change-Id: I73cfc9e97a2291ff6c81f554268173838b0cd663
Reviewed-on: https://go-review.googlesource.com/24922
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed